The negative ends of two magnets repel each other. The negative end of one magnet attracts the positive end of another magnet. This same idea affects the retention of plant nutrients in soil. Some plant nutrients are cations, which have a positive charge, and some are anions, which have a negative charge.

Soil particles are similar to a magnet, attracting and retaining oppositely charged ions and holding them against the downward movement of water through the soil profile. Soils with a strong charge have the ability to hold more nutrients.

• 1/2 cup bentonite clay
• 6-volt or 9-volt battery
• 2 wires with alligator clips
• Glass jar or measuring cup

1. Thoroughly mix 1 part bentonite clay with 2 parts water.
2. Attach one alligator clip to the negative post on the battery. Put the wire end into the soil slurry.
3. Attach the other alligator clip to the positive post and put the wire into the soil slurry.
4. Make a prediction about how the soil with react with the different wires.
5. Wait for 45 minutes and then pull the wires out of the soil slurry.

If you prefer to do this activity yourself, ask an adult and use this PDF for instructions.

1. Was your prediction correct?
2. Which wire had the most soil attached to it? Why do you think this is?
3. What does this mean for soil and its nutrient holding capacity?
4. Would a soil with a greater negative charge be able to typically hold more nutrients?
5. What do you think this means for managing your soil for growing plants?

Extra Reading 

Too Much of a Good Thing: Nitrogen Leaching

Just like magnets, negative charges repel negative charges. Soils with a high negative charge tend not to hold negatively charged nutrients. As a result, water moving through the soil profile will leach negatively charged nutrients, such as chloride, nitrate, and sulfate out of the root zone. This leaching can result in contamination of groundwater, streams, and lakes or have other environmental implications.